Blue Sky Science: How does the moon affect the tide?

February 19, 2019

Q How does the moon affect the tide?

— Ellie Webb, Mineral Point, Wis.

A Grant Petty, professor in the department of atmospheric and oceanic sciences at the University of Wisconsin-Madison:

Gravity helps make the ocean tides happen. Gravity is the attractive force that all matter exerts on other matter, and it’s what holds us to the Earth. All the matter that makes up the Earth attracts each other, and the result is that it pulls itself into this round ball.

If there were no other external factors, the Earth would be a smooth, round sphere, and the oceans would be the same depth everywhere and would never change. But our moon exerts its own gravitational force that affects some of our life on Earth.

The tides are the result of the moon exerting its gravitational force on the ocean and bulging it both toward and away from the moon. The tide is higher, the ocean is higher, at the location closest to the moon and on the opposite side of the Earth.

As the Earth rotates, the position relative to the moon changes, so the bulge moves. Every six hours the tide goes from high to low, and then it goes from low to high again the next six hours. Over 24 hours you typically see two high tides and two low tides.

The sun can also be influential.

The sun, if it happens to be aligned with the moon, also exerts a gravitational force and can work together with the moon to make the tides stronger. If the sun is unaligned with the moon, then it works against the moon, reducing the high tide and raising the low tide a little bit so the peak and the valley are not as large.

The process is more complicated as the oceans have a shape, different depths and coastlines, so the water sloshes around in ways that cause the sea level to rise and lower by different amounts depending on where you are.

Some locations, like the Bay of Fundy in Canada, see a sea level variation from low to high by as much as 50 feet. In most of the world, the variation is only a few feet up and down.